The present invention relates to conveying systems and more particularly to the cleaning or conditioning of impermeable belts that operate with paper conveying systems.
During a papermaking process, a slurry is placed on a forming fabric or press fabric to form a fibrous web of cellulosic fibers at a forming section of a paper machine. Water is drained from the slurry in the forming section to form on the press fabric a fibrous web that includes paper fibers from the slurry. The newly formed web is then conducted to a press section. The press section includes a series of press nips. The press nips subject the fibrous web to compressive forces. Those forces are applied to further remove water from the web by pressing the water into the press fabric, which absorbs and holds the water. The web is then conducted to a drying section, which typically employs drying drums around which the fibrous web is conveyed. The drying drums also reduce the water content of the web to a final desirable level through evaporation, yielding a paper product that can be cut or otherwise processed and packaged.
It is desirable to remove as much water from the web as possible through mechanical processes, such as via the press rolls. The drying sections consume a large amount of energy. The dryer drums are in many cases heated from within by steam. Energy costs associated with steam production can be substantial and provide one factor mitigating against extensive use of the drying section. Attempts have therefore been made recently to remove as much water as possible through mechanical pressing as opposed to evaporation.
Traditional press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. Increased demand has mandated that the paper machines be run at higher speeds, including increased web speeds. Increasing the web speed however decreases the amount of time that the web spends between the press nips, tending to render press drying less effective. While the pressure applied by the press nips can be increased, there are limits to the amount of pressure that the fibrous or paper web can be subjected.
One solution to the above-described dilemma in recent years has been to use longer press nips, one type of which is known as a “shoe” type of press nip. The longer shoe press nips are advantageous with respect to paired nip rolls because the longer press nips subject the web to pressure for a longer duration and thus remove more water from the web. Consequently, less water remains to be removed through evaporation in the dryer section.
Shoe type press nips include a cylindrical press roll and an arcuate pressure shoe. The shoe has a concave surface with a radius of curvature close to that of the cylindrical press roll. The roll and shoe when brought together form a press nip in which the length of mating surfaces may be five to ten times longer than similar sized cylindrical press roll nips. Since the mating surface length may be five to ten times longer, the so-called dwell time, during which the fibrous web is under pressure in the longer nip, is correspondingly longer that it would be in a two cylindrical roll press. The newer long nip technology has dramatically increased the amount of dewatering caused by the press section of the paper machine.
The long nip presses of the shoe type use a particular belt designed to protect the press fabric that supports, carries and dewaters the fibrous web. Without the belt, the press fabric would be subject to excessive and accelerated wear due to the long dwell time resulting from direct, sliding contact over the stationary pressure shoe. The protective belt is provided with a smooth, impermeable surface or coating that slides over the stationary shoe on a lubricating film of oil. The impermeable belt moves through the nip at roughly the same speed as the press fabric, subjecting the press fabric to a minimal amount of ribbing against the surface of the impermeable belt.
One method of making impermeable conveying belts is to impregnate a synthetic polymeric coating onto or into a woven base fabric formed into an endless loop. The coating typically forms an impermeable layer of some predetermined thickness on at least the surface of the belt contacting the arcuate pressure shoe to protect the woven base fabric from the shoe. The coating has a smooth, impermeable surface that slides readily over the lubricated shoe and prevents any of the lubricating oil between the coating and the shoe from penetrating the structural fabric of the belt and contaminating the press fabric and the fibrous web.
Besides enabling the machines to run faster by increasing the “dwell time” between nip rolls, certain paper machines today are attempting to increase productivity by closing the “draw” between the press section and the drying section. In the past, the paper web was practically fully unsupported for about 1.0 m to 2.0 m in the area between the press and the dryer sections. Such unsupported area of the web became exposed to strong air currents. One reason the draw was necessary was to detach the web from the center roll. The web fluttering in the free, unsupported area was controlled by arranging a high difference in speed in the area between opposing rolls to thereby pull the web tighter.
The closed draw concept was developed to address a problem, namely, that the paper web was tensioned highly at its weakest point, the unsupported area between the press and dryer sections. In the closed draw concept, the dryer fabric is brought as near to the press section as possible. By minimizing the length of the free, unsupported web transfer from the press section to the first dryer, the fluttering of the web can be significantly reduced or eliminated totally. The formerly needed high-speed differential is now significantly reduced because the remaining draw is merely needed to pull the web off the press roll surface.
As modern high-speed paper machines approach speeds of 1900 meters per minute, increasing the force needed to release the sheet from the press section, the tension in the web in the open draw section will be further increased. At some point the web will not be able to withstand the forces imposed. Consequently, the closed draw concept appears to be important for the future of high-speed paper machines.
New press section designs, such as the Valmet OptiPress® from Metso Paper provide total sheet support with no open draws. That system however, especially when run with four felts, can lead to a significant amount of rewet caused by moisture being conveyed back to the web by saturated felts. To reduce rewet and improve sheet handling, one of the bottom press felts can be replaced with a non-porous, water impermeable transfer fabric. One such belt is a TransBelt® belt from Albany International Corp., Albany N.Y. That belt includes a woven substrate and a multi-component polymer layer placed onto the paper or face side of the belt. The polymer coating is well-suited at adhering and then releasing from the web at high speeds.
The discussion above describes two instances where the press fabric has been coated with a water impermeable coating, such as a polymer coating. In the first instance, for operating with the arcuate pressure shoe, the coating is applied to non-paper or back-side of the belt as installed. In the second instance, for reducing rewetting in a closed draw system, the coating is applied to the paper or face-side of the belt as installed in the paper machine.
Because the above-described water impermeable belts for the above-described systems are relatively new, not much is known about the conditioning needed for such belts. Typical fabrics used to support the web, such as the wires of the wet end and the dry end of the web and the felts of the press section, will absorb fiber material and impurities that gradually block the fabric and prevent water from migrating through the fabric, if the fabric is not cleaned from time to time. Conditioning devices have therefore been used with water permeable fabrics, for example, in the fabric return loop in to clean the fabric as it passes over the guide roll or in the fabric return loop to clean the fabric as it passes over the guide roll or similar apparatus.
To date, it does not appear that water impermeable belts have been cleaned using chemical solutions. Published PCT application WO 98/45534 (PCT/F198/00288) discusses a transfer belt which is “water non-receiving” and that withstands intensive cleaning, for example, by high-pressure water jets or doctors. Further, literature for the TransBelt® belt states that light doctoring and a fan lubrication shower on the surface of the TransBelte belt are all that is required to maintain a good working condition of the surface.
The inventors of the present invention believe that impermeable belts accumulate enough deposits to warrant chemical conditioning. The present invention addresses that need.